FeatureFlagging

Thin wrapper around LaunchDarkly for setting up and accessing feature flags.

Installation

Add this line to your application's Gemfile:

gem 'feature_flagging'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install feature_flagging

Then add it to the relevant initializers:

launchdarkly_config_hash = YAML.load(File.read("launchdarkly.yml"))
FeatureFlagging::Configuration.start_client(launchdarkly_config_hash[:sdk_key])

For example in config/puma.rb:

launchdarkly_config_hash = YAML.load(File.read("launchdarkly.yml"))
FeatureFlagging::Configuration.start_client(launchdarkly_config_hash[:sdk_key])

In config/unicorn.rb:

after_fork do |_server, _worker|
  launchdarkly_config_hash = YAML.load(File.read("launchdarkly.yml"))
  FeatureFlagging::Configuration.start_client(launchdarkly_config_hash[:sdk_key])
end

In config/initializers/feature_flagging.rb for all tests:

if Rails.env.test?
  launchdarkly_config_hash = YAML.load(File.read("launchdarkly.yml"))
  FeatureFlagging::Configuration.start_client(launchdarkly_config_hash[:sdk_key])
end

Upgrading

To upgrade to the latest version on main, run:

$ bundle update feature_flagging

Usage

Adapter over this gem

You may want to create a slim adapter over this FeatureFlagging gem to be the entry point for using flags. It knows how to build the user information to send to the feature flagging service.

class FeatureFlags
  def self.evaluate(key, default)
    new.evaluate(key, default)
  end

  def self.all_flags_state
    new.all_flags_state
  end

  def evaluate(key, default)
    FeatureFlagging::FeatureFlags.evaluate(key, user_info, default)
  end

  def all_flags_state
    FeatureFlagging::FeatureFlags.all_flags_state(user_info)
  end

  private

  def user_info
    Current.user ? signed_in_user_info : anonymous_user_info
  end

  def signed_in_user_info
    {
      key: Current.user.uuid,
      anonymous: false,
      custom: {
        evaluation_datetime: Time.now.utc.to_i * 1000,
        tenant_id: Current.tenant.uuid
      }
    }
  end

  def anonymous_user_info
    {
      key: UuidAdapter.generate_uuid,
      anonymous: true,
      custom: {
        evaluation_datetime: Time.now.utc.to_i * 1000
      }
    }
  end
end

Tests

In your spec_helper, make sure you reset the SpecStore:

RSpec.configure do |config|
  config.before do
    FeatureFlagging::SpecUtils.reset
  end
end

Then use it like this in a spec to set up a feature flag in memory:

class SomeFeatureFlagDependentService
  def run
    if FeatureFlags.evaluate(:some_feature_flag, "bad_variation") == "good_variation"
      "good"
    else
      "bad"
    end
  end
end

RSpec.describe SomeFeatureFlagDependentService do
  it "returns the OFF variation when that is received" do
    FeatureFlagging::SpecUtils.set_flag_variation(:some_feature_flag, "good_variation")
  
    result = SomeFeatureFlagDependentService.run
  
    expect(result).to eq("good")
  end
end